The minimum post to earn merit is one post.
This is not by default, else everyone making a single post must earn merit.
While that one post does not guarantees for a merit.
That same one post must be a quality one.
You may also have to make more than one post before you see someone to merit you.
You can make one post and earn several merits if the post is very useful and helpful. You can make hundreds of posts and no one will send merit to the posts because they are useless or of low quality.
Its also good and very important to be reminded of how to make a quality post on this heading.
https://bitcointalk.org/index.php?topic=5184741.0#post_altgiveaway